FORMLABS WHITE PAPER: Engineering Fit: Optimizing ......stereolithography (SLA) has a single automated production phase. Proper dimensional tolerancing lowers post-processing time

May 2017

FORMLABS WHITE PAPER:

Engineering Fit: Optimizing Design

for Functional 3D Printed AssembliesTolerance and fit are essential concepts that engineers use to optimize the functionality of mechanical

assemblies and the cost of production. Formlabs extensively studies the accuracy of our materials and

works to maximize repeatability between prints and across printers. The data and best practices in this

white paper can help Form 2 users to design functional assemblies that work as intended, with the least

amount of post-processing or trial-and-error.

FORMLABS WHITE PAPER: Engineering Fit: Optimizing Design for Functional 3D Printed Assemblies 2

Table of Contents

Value of Tolerances in 3D Printing . . . . . . . . . . . 3

Fit Selection. . . . . . . . . . . . . . . . . . . . . . . . 4

Measuring and Applying Tolerance. . . . . . . . . . . 6

Friction. . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Lubrication . . . . . . . . . . . . . . . . . . . . . . . . 11

Bonded Components. . . . . . . . . . . . . . . . . . . 11

Machining Printed Parts . . . . . . . . . . . . . . . . .12

Conclusion . . . . . . . . . . . . . . . . . . . . . . . .13


Value of Tolerances in 3D PrintingIn traditional machining, tighter tolerances are exponentially related to increased

cost. Tighter tolerances require additional and slower machining steps than wider

tolerances. Machined parts are designed with the widest tolerances allowable for

a given application.

Unlike machining, where parts are progressively refined to tighter tolerances,

stereolithography (SLA) has a single automated production phase. Proper

dimensional tolerancing lowers post-processing time and ease of assembly, and

reduces the material cost of iteration. Improper tolerances for a particular material

can also result in broken parts, especially for press-fit components in brittle materials.

Tolerance is the predicted range of possible

dimensions for parts at the time of manufacture.

Post-processing steps for printed assemblies include cleaning, sanding supports,

and lubrication. Sanding an active surface is a reasonable method for achieving the

correct fit if the part is a one-o!, because less tolerancing work is required in the

design phase. With larger assemblies, or when producing multiples of something,

proper dimensional tolerancing quickly becomes worthwhile.

ACTIVE SURFACE Model region where two surfaces touch and either move

against each other or have a static fit.

With larger assemblies, or when producing

multiples of something, proper dimensional

tolerancing quickly becomes worthwhile.

ShaftHoleBasic Size


Fit SelectionThe functional needs of the assembly will define how parts should fit together.

Free movement of a component requires clearance, or space between the active

surfaces. This is achieved by ensuring that the tolerance zones of the active

surfaces do not overlap. If no motion between parts is needed, a transition fit will

allow for easy assembly and disassembly. A transition fit has partially overlapping

tolerance zones.

An interference fit provides a rigid, strong connection, but requires much more

force applied in assembly.

Note: Interference fit has fully overlapping tolerance zones and requires the use of

a resin with greater elongation such as Durable, Tough, Flexible, or Standard.

Small amounts of deviation in manufacturing means that engineering fit is a

continuum rather than three completely separate stages. Larger clearance fits

trade precision for freedom of movement. Tighter transition fits are stronger, but

cause more wear on the connection. An interference fit that requires more force to

join will be more challenging to disassemble.


Fit can be divided into three categories–clearance, transition, and

interference–and each category is defined by two types of fit.

CLEARANCE A sliding fit will have some lateral play, while a running fit will have

almost no play. A running fit has slightly more friction, but more

accurate motion.

TRANSITION With a keying fit, a component will accurately insert into or around

another part, with only a light force needed to install and remove it.

A push fit will require more force to join and remove the parts, but

it will still be possible by hand.

INTERFERENCE A force fit requires substantial force, likely with additional hand

tools like a hammer to install, and is not intended to be removed.

A press fit will need much more force applied by a tool such as

an arbor press to install.

PLAY The amount of space for movement in an unintended direction

within a mechanism.

Fit is a spectrum that can be divided into three categories, which help dial in fit based on the needs

of your assembly. Color coding in this spectrum is applied to the graphs below.


Measuring and Applying ToleranceFormlabs examined a variety of common geometries to find the real-life tolerance

needs of each kind of fit. The two SLA materials tested were Tough and Durable

Resin, which were designed to be used to print functional product prototypes

that undergo mechanical and tribological (friction) stress. The following examples

demonstrate why it is beneficial to apply the correct material to each application.

Fit conditions in each graph are color coded: blue is a clearance fit, green is a

transition fit, and orange is an interference fit.

HOLE AND SHAFT

A hole and shaft will usually require a clearance condition, which may range from

a sliding fit to a running fit depending on needed accuracy. A running fit will require

sufficient lubrication for free movement. The hole and shaft print is a useful general

test of fit conditions for both Durable and Tough Resins.

ALLOWANCE (MM)

RE

MO

VA

L F

OR

CE

(N

)

-0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

100

80

60

40

20

0

Tough Results: 0.05 mm or greater of allowance resulted in

a clearance fit between the hole and shaft. Between 0.0 mm

and 0.0375 mm resulted in a transition fit—the shaft could

be inserted with a normal amount of hand pressure. Below

0.0 mm (an interference fit), the shaft becomes much harder

to insert, and quickly exceeded the 55 N limit of the force

meter. The shaft was able to be inserted with an arbor press at

-0.0375 mm, but could not be removed.

Durable Results: In this test, the Durable part transitioned from

a transition fit to a freely insertable clearance fit at 0.0625 mm

of allowance. At 0.0125 mm of allowance and below the parts

had an interference fit and were difficult to remove. However,

the parts could still be joined even at a negative clearance of

-0.0375 mm. This is due to the high elongation of Durable Resin.

Download Hole & Shaft Test Model


BALL AND SOCKET

A clearance condition should exist to allow the ball to rotate freely in the socket.

However, there is a large interference between the radius of the ball and the

socket opening. The socket opening needs to deform enough to be inserted, but

not come out in normal use.

Durable Results: In this test there was an inflection point between 0.2 mm and 0.3

mm of allowance where the ball and socket becomes easier to separate. Between

0.0 mm and 0.2 mm, the ball moves smoothly in the socket but stays in position. The

interference fit in this range would be ideal for articulated figurines, which need to

keep their orientation. Above 0.3 mm, the ball is slightly loose and moves freely in

the socket. Below 0.0 mm, the ball could not be inserted at all.

Tough Results: The ball and socket model was also printed in Tough Resin, but

could not be inserted by hand because of the high friction and high strength of

the material. Durable is better suited for a ball and socket geometry due to its

wear resistance.

Download Ball & Socket Test Model


ROD AND BUSHING

A bushing is a type of plain bearing designed for smooth, free movement along a

rod. There should be a clearance fit between the rod and the bushing. Depending

on the application, the clearance may be larger or smaller.

Durable Results: Bushings printed in Durable Resin had smooth and low-friction

movement over the polished steel guide rod. With a designed allowance of 0.025

mm, the bushing showed a smooth running clearance fit with no play. At and above

0.025 mm, the bushing did not grip the rod at all, allowing it to slide freely. Between

0.0 mm and 0.025 mm, the bushing required a light force to push it over the rod.

Below 0.0 mm of clearance an interference fit was observed that required significant

force to slide the bushing over the rod, although the rod was insertable due to the

compliance of the material. In this range the part isn’t functional as a bearing.

Correctly lubricated, Durable Resin is a good choice for quickly prototyping custom

bushings, gear systems, and smooth polypropylene-like products with moving parts.

For kinetic assemblies that need to last for many motion cycles without wear, it is

best to to use o!-the-shelf Delrin bushings embedded in Tough Resin parts.

Note: Tough Resin is best suited for creating strong structural components that do

not need to experience long term friction.

Download Rod & Bushing Test Model


Friction

The amount of friction force between two components is the product of the

force on the mating surface (directly related to fit), and a constant (the coefficient

of friction), which is specific to each material. Coefficient of friction is useful for

predicting how much resistance your parts will have to movement and wear, and

how you can expect Formlabs resins to perform relative to other common materials.

Formlabs tested coe!cient of friction using a weighted sled, a track, and a force meter.

Below are the unlubricated coefficients of friction between the following pairs of

materials printed with 100 micron settings:

Parts were measured without any post-process smoothing or sanding. Durable

Resin has the lowest sliding friction of Formlabs resins due to its high inherent

lubricity. By contrast, Tough Resin has a higher sliding friction and markedly higher

static friction which caused stick-slip behavior in our tests. Durable Resin’s lower

coefficient of friction makes it best suited for moving components which interact

in kinetic assemblies.

Sliding components such as rails, pistons, and rods have lower friction if the contact

surface area of the two mating surfaces is reduced. This is achieved by orienting

objects in PreForm so that the layer “grain” pattern is perpendicular between the

parts. If the grain is parallel the layer grooves will mesh, creating more surface area

and higher static and kinetic friction.

Coefficient of Static Friction (µs) Coefficient of Kinetic Friction (µ

k)

Durable | Durable 0.32 0.14

Tough | Tough 0.77 0.2

Durable | Tough 0.57 0.16


Formlabs tested static and kinetic coefficient of friction between Durable components

with parallel and perpendicular grain patterns by orienting the the sled 90° in Preform.

In both the static and kinetic tests, the perpendicular orientation had lower frictional

coefficients. The coefficient of static friction is impacted more significantly by

grain orientation: in our tests, friction between perpendicular grain surfaces was

43 percent lower than parallel grain surfaces. Kinetic friction was 11 percent lower

between perpendicular grain surfaces.

Friction between parts decreases over time as the surfaces experience wear. This

is often beneficial to kinetic assemblies, and sanding and polishing is a deliberate

example of wear. However, excessive wear tends to increase clearances between

parts. Lubrication is the best way to reduce long term wear.

Note: Durable Resin is designed to be the most wear-resistant Formlabs material,

and is most suitable for for gears, bushings, and kinetic assemblies.

In some cases—such as rollers, wheels, and robotic grippers—more friction is

beneficial. For these applications, Flexible Resin has a higher coefficient of friction

and lower lubricity.

Most static and kinetic friction High static and moderate kinetic friction

A micro-scale diagram of friction between surface orientations.

Least static and kinetic friction


LubricationLubricants are essential to keeping components running smoothly in kinetic

assemblies. Mineral oil is an inexpensive and commonly available lubricant

commonly used with SLA prints. Silicone oil based lubricants, such as Super Lube®,

also work well, and last longer without becoming sticky.

Bonded ComponentsIn order to bond printed components with adhesives, a narrow clearance fit is desired.

Cyanoacrylate (Super Glue) will fill thin gaps due to its low viscosity. A syringe of resin

cured by hand with a UV or blue-violet (405 nm) laser pen (and UV safety goggles) can

be used to weld parts together in butt joints.

Air-powered, functional scale model of a flat two-cylinder internal combustion engine printed in Durable

and Tough Resin and lubricated with mineral oil.


Machining Printed PartsThe most common post-processing steps for printed assemblies are sanding,

polishing, and lubricating. Occasionally it can be useful to machine a plastic part

after printing, for example, if the tolerances of a feature need to be tighter than 0.025

mm, or to alter a feature after printing. Adding holes with a drill press or threads with

a tap can be faster and more efficient than re-printing if the tools are available and

the design has changed in the middle of a print.

Tough and Durable Resin withstand machining the best of Formlabs range

of materials due to their high strength and elongation. Other Formlabs Resins

can also be machined, though they require more conservative techniques and

faster tool speeds.

Which machining operations work best with SLA resins?

High Temp

Sanding

High-speed fly cutting

High-speed, small-diameter boring and reaming

Tough, Durable,

Standard

Sanding

Face and peripheral milling

Drilling

Boring

Reaming

Tapping

Flexible

Sanding

Drilling

Cutting


ConclusionSpecifying fit based on material properties and mechanical function is a necessary

practice in product engineering. The fit ranges shown for common geometries can be

broadly applied to many designs to achieve functional prototypes with fewer iterations.

For even more accuracy and an intuitive understanding of how parts will fit together,

print the test models in a wider variety of materials and see how they perform.

Beyond fit, choosing the right material is essential for creating working prints. Formlabs

materials vary significantly in tensile strength, elongation, and wear resistance. Some

basic geometries such as the rod and bushing will work significantly better in Durable

Resin due to the high wear resistance and low friction of the material. Structural

components under load would be most successful in Tough Resin, which has high

tensile strength and a flexural modulus similar to ABS plastic.

Download our Full Materials Data Sheet

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FORMLABS WHITE PAPER: Engineering Fit: Optimizing ......stereolithography (SLA) has a single automated production phase. Proper dimensional tolerancing lowers post-processing time